From the online dictionary:
a mixture of dark bituminous pitch with sand or gravel, used for surfacing roads, flooring, roofing, etc.
The ratio of sand/gravel is a variable, even the type of gravel is a variable, depending upon which part of the country/or which country you live in. So all asphalt is not created equal. This also explains the widely varied mileage on a set of the same tires in various parts of the country.
If you're not living on the edge, you're taking up too much space.